Arginine is one of the predominant amino acids present in grape musts (Henschke and Jiranek, 1993). Arginine is thought to be transported into the yeast cell by the general amino acid permease encoded by the GAP1 gene (Jauniaux and Grenson, 1990) or by the arginine permease encoded by the CAN1 gene (Hoffmann, 1985). In Saccharomyces cerevisiae, arginine is reportedly degraded into urea and ornithine by arginase, the product of the CAR1 gene (Middelhoven, 1964; Sumrada and Cooper, 1982).
The DUR1,2 gene encodes a bifunctional enzyme, urea carboxylase-allophanate hydrolase (Dur1,2; urea amidolyase) which can degrade urea to ammonia and CO2. The urea carboxylase function is encoded separately in the enzyme from green algae, which catalyzes the reaction: ATP+urea+CO2=ADP+phosphate+urea-1-carboxylate (EC 6.3.4.6; systematic name: urea:carbon-dioxide ligase (ADP-forming); other name(s): urease (ATP-hydrolysing); urea carboxylase (hydrolysing); ATP-urea amidolyase; CAS registry number: 9058-98-4; Roon et al., 1970; Roon and Levenberg, 1972; Sumrada and Cooper, 1982). The allophanate hydrolase function is also encoded separately in the enzyme from green algae, which catalyzes the reaction: urea-1-carboxylate+H2O=2 CO2+2 NH3 (EC 3.5.1.54; systematic name: Urea-1-carboxylate amidohydrolase; Other name(s): allophanate lyase; CAS registry number: 79121-96-3; Maitz et al., 1982; Roon, et al., 1972; Sumrada and Cooper, 1982).
In S. cerevisiae, the DUR1,2 gene is subject to nitrogen catabolite repression (NCR) by preferred nitrogen sources present in grape must (Genbauffe and Cooper, 1991). Urea that is not degraded may be secreted by yeast cells into the fermenting grape must. Secreted urea can react with ethanol in the must to form ethyl carbamate, which has been shown to produce various benign and malignant tumours in a variety of experimental animals (Mirvisch, 1968) and may therefore be considered a potential health risk to humans.